The present invention relates generally to temperature sensing devices. More particularly, the present invention relates to temperature monitoring systems employing temperature sensors having non-linear electrical response characteristics with temperature and circuitry for linearization and stabilization of these characteristics. More specifically, the present invention concerns apparatus and method for the linearization, stabilization or substantial elimination of other deleterious non-linear variations with temperature of systems utilizing thermistor temperature sensors.
Thermistor temperature sensors have long been recognized for their utility in accurately and precisely monitoring a wide range of temperatures. The thermistor's usefulness as a temperature sensing device derives from its characteristic very large change in resistance with temperature. Unfortunately, this change is also highly non-linear, as evidenced by the following first order mathematical approximation of a thermistor's resistance with temperature: ##EQU1## where R.sub.T represents the thermistor resistance at any absolute temperature T; R.sub.T.sbsb.o represents the thermistor resistance at absolute reference temperature T.sub.o ; and .beta. (also referred to as "beta") represents the thermistor material constant. Thus, in order to effectively employ a thermistor in an accurate temperature monitoring system, the circuitry utilized in association with the thermistor must be either capable of following this non-linearity or, as done in essentially all commercially practical devices, assume absolute linearity with temperature for each incremental temperature range.
Accordingly, prior art devices have developed numerous linearization techniques to insure measurement accuracy. The majority of these techniques are based upon trading precision for accuracy (or, in other words in such cases, sensitivity for linearity), by placing additional resistances in series and/or parallel with the thermistor to reduce its rate of change of resistance with temperature [i.e., the curvature of the thermistor's Resistance-Temperature (or R-T) characteristic].
One prior art reference discloses a different linearization technique in which a time variant signal, whose characteristics are the inverse of those of a non-linear temperature sensor, is periodically generated and compared with that of the sensor output. Upon coincidence in the value of the two signals the comparator emits a short pulse of constant duration. Because the two signals are the inverse of each other, a measurement of the time from the start of the comparison to the time at which both signals have equal value--the so called "intersection time"--yields a linearized indication of the sensor's temperature. Although this technique does serve to achieve a highly linearized signal with temperature, when taken by itself such a device still suffers from serious loss of accuracy and precision in measurement, and requires unnecessary, additional, complex circuitry.
One reason for these deficiencies results from the fact that the value of beta (the thermistor material constant) has been assumed to be absolutely independent of temperature in all devices employing non-linear, semiconductor temperature sensors of which I am aware. In fact, especially over a large temperature range, substantial variations in the value of beta occur, resulting in an appreciable loss of both accuracy and precision.
A second deficiency in the performance of thermistor temperature sensing devices such as that described above stems from error induced by the heat generated within the thermistor itself as the signal current passes therethrough. One approach taken by others to mitigate the effects of self-heating has involved the intermittent operation of the thermistor sensor. But such periodic energizations reduces the frequency with which temperature samplings can be made by increasing the time necessary for each measurement, as well as necessitating further complex circuitry added to the system.
This approach is not the only source of unnecessary complex, circuitry in the above offsetting signal linearization scheme. As previously explained, after the variable intersection time is reached, a coincidence pulse of constant duration is generated to terminate this period and initiate temperature computation and display. A "controller" must be provided to initiate the temperature computation and to subsequently begin a new measurement cycle. Moreover, the necessity to manually restart each such cycle severely limits the rapidity with which successive measurements can be made.
I have found several techniques and circuits for significantly improving both the accuracy and precision of temperature systems having sensors with non-linear characteristics. Generally, the present invention contemplates the utilization of a time exponential function to offset the temperature dependent exponential function characteristic of thermistor sensors described by equation (1). More specifically, I have found an extremely simple current division network which, when used in conjunction with thermistor sensor output signal amplifiers, substantially reduces or eliminates the aforementioned variations in the value of beta, regardless of temperature.
Additionally, I have found that by impressing a particular feedback signal upon the thermistor sensor in place of the conventional constant voltage power source, the power dissipated by the thermistor itself may be fixed so as to substantially avoid the self-heating phenomena without having to periodically operate the same. Beyond this, by proper selection of this feedback signal, the thermistor output signal level may be compressed to such an extent that the linear response range of a temperature measuring system having a thermistor sensor may be substantially (and, at least theoretically, infinitely) expanded beyond that of all presently known systems.